Commissioning Makeup-Air Units

Once a makeup-air system is designed and installed, it is time for start-up. Commissioning is a particularly important step with makeup-air units due to the potential problems with pressure imbalances within the building.

Commissioning indirect-fired makeup air units or those with hot water or electrical heating coils is a fairly straightforward procedure similar to that of a standard rooftop unit or air handler.

The first step is to assemble the proper instruments: a voltage meter, an amperage meter, a U-tube manometer, a tachometer, and a thermometer. These are the most commonly required instruments, but also check the start up instructions for the particular unit installed.

Most manufacturers include a start-up documentation sheet that should be filled out and submitted along with the commissioning report and included with the operations and maintenance manuals. Be certain to write down all of the nameplate information.

COMMISSIONING CHECKLIST

Make sure that the fan wheel rotates freely without rubbing on anything, and check the other elements of the drive train to be sure that everything is aligned and in good working order.

The voltage must be checked before starting the unit. Also, if the motor is three-phase, check to see if the phases are connected in the proper order.

Turn on the fan to see that it is rotating in the proper direction. Then, measure the fan speed, the actual voltage delivered, and the actual running load amperage. On a three-phase motor, record the amperage on each phase.

Measure the air delivery and compare it to the design. Typically, if it is not within ±5 percent of design, you will need to determine why that is and make the appropriate adjustments. This may require the assistance of a test and balance agency.

Before activating the burner, check the gas pressure at the manifold. If this is within specs, start the burner and check the gas pressure again under operating conditions. Measure the discharge temperature and check the operation of the controls.

These are the most important steps to take when commissioning an indirect-fired unit, though be certain to follow the manufacturer's start up instructions all the way through.

Direct-fired makeup air units require a little more attention to detail in some of the steps. One of the most important is making sure that the unit is producing the proper airflow. With direct-fired units, too much airflow can result in combustion problems and too little airflow can produce a longer than normal flame, which can overheat the fan bearings.

Direct-fired units have two basic designs. In one design, the entire airflow of the unit passes over the burner. The airflow in this type of unit is attuned by means of adjustable baffles above and below the mouth of the burner (Figure 1). The pressure drop across the burner must be within a narrow range to provide proper combustion. See the manufacturer's literature for the proper settings.

The other basic design is a face-and-bypass arrangement where only a portion of the total unit airflow passes over the burner. In this type of unit, the airflow over the burner is adjusted at the face-and-bypass damper.

Due to the large turndown ratio available in direct-fired units (see the first article in this series, “Choosing the right makeup-air unit,” Nov. 2004), proper gas pressure is very important. An undersized gas pipe connecting the unit to the gas main can result in starving the burner on high-fire.

If the makeup air unit is being tied into an energy management system that will control it, the controls contractor has to be made aware of the requirement for a fast-stroking gas valve. A direct-fired unit can go from low fire to high fire in a couple of seconds. Most controls contractors are more used to dealing with stroke times of 30 sec or more. Installing a slow-stroking valve operator will result in callbacks.

ACKNOWLEDGEMENT

I'd like to thank Jim Dirkes and Ken Ignasiak with Rapid Engineering for their help in preparing this article.

A member of HPAC Engineering's Editorial Advisory Board, Robert W. Tinsley PE, CFPS, CIAQP, has authored numerous articles for HPAC Engineering on the topics of insulation, sound attenuation, and commissioning. He can be reached atbtinsley@p2rscos.com.